Three causes, and I check them in this order: geometry, focus uniformity, installation drift.
Geometry first, because it is the commonest and the most self-inflicted. If the projector is not square to the screen and the shape has been "fixed" with keystone correction, you traded geometry for sharpness. Digital keystone rescales the image, throwing away pixels along the way, and it softens the result unevenly, so one side or the corners suffer most. The proper fix is physical alignment: projector centred on the screen, lens axis perpendicular, keystone at zero. If your model has optical lens shift, use that for the last few centimetres; it moves the lens, not the pixels, and costs nothing in sharpness.
Focus uniformity next. Focus on a test pattern or the menu text at the corners, not just the centre. Some lens and distance combinations cannot be sharp everywhere at once, and you find the compromise point deliberately rather than by accident. If the picture is sharp on one side only, that is very often a projector rotated slightly around its vertical axis, so one edge is nearer the screen than the other; back to geometry.
Installation drift last. Ceiling mounts sag, wall brackets settle, and a projector that was sharp on day one and soft on one side today has usually moved a few millimetres. Nobody notices the mount tilting; everybody notices the picture. Re-level, re-square, refocus.
Edges still soft with zero keystone, correct alignment and careful focus? Then the lens itself may have an issue: a decentred element or a focus mechanism that no longer holds. That is authorised service, and sooner rather than later if it is under warranty.
The setup rule that avoids all of this: place first, correct last. Keystone is the last resort, not step one, and a projector that needs a lot of it is in the wrong place. Send us a photo of the projector position and the screen and we can usually see the problem before you climb the ladder.
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